1. Field of the Invention
The present invention relates to a light deflecting unit for deflecting a light beam and, more particularly, to a light deflecting unit such as a light beam scanner utilized in a xerographic printer for deflecting a laser beam.
2. Description of Related Art
Generally, a xerographic printer for recording image information supplied from a host computer is well known as an apparatus in which a light deflecting unit is utilized. Hereinafter, a conventional light scanning device of the xerographic printer is described with reference to FIG. 19.
A light scanning device 102 for scanning a photosensitive drum 103 as a recording medium with a laser beam includes a laser unit 123, a light deflecting unit 129 and a beam detecting unit 158. The laser unit 123 has a semiconductor laser 148 for generating a laser beam and a collimator lens 149 disposed integrally with the semiconductor laser 148. The semiconductor laser 148 emits a laser beam modulated on the basis of image data. The laser beam from the semiconductor laser 148 passes through the collimator lens 149 and becomes a collimated beam coincident with an optical axis of the collimator lens 149. The laser beam emitted from the collimator lens 149 is conducted to an incident aperture of a beam expanding optical system 125, whereby it is expanded in diameter as the collimated beam. The laser beam emitted from the beam expanding optical system 125 converges in a direction orthogonal to a horizontal plane, and the thus-converged beam is incident on the light deflecting unit 129.
The light deflecting unit 129 is provided with a polygon mirror 126 having eight mirror surfaces and a motor 146 having an output shaft. The polygon mirror 126 is fixed to the output shaft of the motor 146 which drives the motor 126 at a high revolution. The output shaft of the motor 146 is supported rotatably by a non-contact type bearing. When the polygon mirror 126 is rotated, a laser beam incident on a mirror surface of the polygon mirror is swept approximately horizontally and is deflected at an equal angular velocity. The laser beam thus deflected by the polygon mirror 126 is focused as a light spot onto the photosensitive drum 103 by means of a known focusing lens 128 having f.theta. characteristics.
The polygon mirror 126 is formed mainly from aluminum by cutting. For increasing the laser beam deflecting speed, it is necessary to rotate the polygon mirror 126 at a high speed. To this end, the motor 146 uses an electromagnetic force and is capable of rotating at a high speed of 10,000 r.p.m. or more, e.g., a known hysteresis synchronous motor or DC servomotor. In the case where the motor 146 is driven at a high speed of 10,000 r.p.m. or more, non-contact type bearings are used as output shaft bearings. A magnetic bearing may be used as a non-contact bearing in the thrust direction, and a pneumatic bearing may be used as a non-contact bearing in the radial direction, as described in Japanese Patent Publication No. 57-49889. If rotation at a high speed exceeding 10,000 r.p.m. is performed using a contact type bearing, e.g., a ball bearing, problems of wear and heat generation of the bearing balls may arise, thus preventing the polygon mirror 126 from being rotated at a high speed and rendering it impossible to increase the laser beam deflecting speed.
The beam detecting unit 158 is provided with a reflecting mirror 155, a slit plate 156 having a small incident slit, and a photoelectric conversion element 157 which is high in response speed, e.g., PIN diode. When a laser beam which has been swept by the light deflecting unit 129 reaches a reference position, it is reflected by the reflecting mirror 155, then passes through the slit plate 156 and is detected by the photoelectric conversion element 157. Upon detection of the laser beam, the photoelectric conversion element 157 produces a detected signal. This detected signal is used to determine an emission start timing of the laser beam from the semiconductor laser 148, beam of which has been modulated on the basis of image data.
The following problems occur in the processing accuracy of the polygon mirror 126 and the mounting accuracy of the polygon mirror 126 to the motor 146. First, if there is an angle division error in the polygon mirror 126, that is, if the mirror surfaces of the polygon mirror are different in length in the scanning direction, a positional deviation in the scanning direction of the laser beam which is focused on the photosensitive drum 103 will arise. Second, if there is an error of parallelism between each mirror surface of the polygon mirror 26 and a rotational axis of the polygon mirror, that is, if each mirror surface is not perpendicular to a horizontal plane, a positional deviation in the direction orthogonal to the scanning direction of the laser beam which is focused on the photosensitive drum 103 will arise, that is, an unevenness in scanning line pitch will occur.
In the above conventional light scanning device 102, a laser beam is detected directly by the beam detecting unit 158 and a reference position is detected for each mirror surface. Even if there is an angle division error in the polygon mirror 126, the emission start timing of a laser beam from the semiconductor laser 148 is adjusted for each mirror surface on the basis of the detected reference position, so that a positional deviation in the scanning direction of the laser beam focused on the photosensitive drum 103 is prevented. Moreover, in the conventional light scanning device 102 a laser beam converges in the direction orthogonal to a horizontal plane by means of the cylindrical lens 127 and is focused on a mirror surface of the polygon mirror 126. Therefore, even if there is a parallelism error of each mirror surface, a positional deviation in the direction orthogonal to the scanning direction of the laser beam focused on the photosensitive drum 103 is prevented. The technique for converging a laser beam in the direction orthogonal to its scanning direction and focusing it on a mirror surface of the polygon mirror, is disclosed in detail in U.S. Pat. No. 3,865,465.
However, in the conventional light scanning device 102 described above, since the motor 146 for rotating the polygon mirror 126 employs an electromagnet, it is necessary to form within the motor 146 a magnetic circuit within the motor 146 including a winding of coil, a permanent magnet, a yoke and a core. Consequently, the size of the motor 146 and hence the size of the light deflecting unit 129 must be increased.
Moreover, in the above conventional scanning device 102 in order to increase the laser beam deflecting speed, non-contact type bearings, a magnetic bearing in the thrust direction and a pneumatic bearing in the radial direction are used as output shaft bearings for the motor 146. On the other hand, since at least one of the constituents of the magnetic circuit, i.e., the winding of coil, the permanent magnet, the yoke or the core, is supported by the rotor integral with the output shaft of the motor 146 which uses an electromagnetic force, the inertia of the output shaft becomes relatively large. Accordingly, for stably supporting such an output shaft having a relatively large inertia, it is necessary to use a large-sized non-contact bearing such as a pneumatic bearing which requires a wide area for supporting the output shaft. Consequently, the light deflecting unit 129 including the bearing mechanism becomes complicated in structure and larger in size, thus leading to an increased cost.